The present invention concerns an analytical test element for determining at least one analyte in a liquid, the use of an analytical test element according to the invention to determine an analyte in a liquid and methods for determining at least one analyte in a liquid with the aid of an analytical test element according to the invention.
So-called carrier-bound tests are often used for the qualitative and quantitative analytical determination of components of liquids and especially of body fluids such as blood. In these tests, reagents and, in particular, specific detection reagents and auxiliary reagents, are embedded or immobilized in suitable layers of a solid support. These layers are referred to as detection elements. In order to determine the corresponding analyte the liquid sample is brought into contact with these detection elements. If a target analyte is present, the reaction of liquid sample and reagents usually results in a signal that can be detected optically or electrochemically and, in particular, to a colour change that can be evaluated visually or with the aid of an instrument, usually by reflection photometry. Other detection methods are for example based on electrochemical methods and detect changes in charge, potential or current.
Test elements or test carriers are often in the form of test strips which essentially consist of an elongate support layer of plastic material and detection elements mounted thereon as test fields. However, test carriers are also known that are formed as small quadratic or rectangular plates.
Test elements for clinical diagnostics are often constructed such that the sample application area and the detection area are stacked above one another in a vertical axis. This type of construction is associated with a number of problems. When the test strip loaded with sample has to be inserted in an instrument such as a reflection photometer for measurement, potentially infectious sample material can come into contact with parts of the instrument and may contaminate them. Hence, a spatial separation of the sample application area and detection element is desirable.
A volumetric metering is often very difficult to achieve in these test elements especially in cases in which the test strips are used by untrained persons, for example, for self-monitoring of blood sugar or coagulation. The transport of the liquid sample from the sample application area to the detection element which is necessary for this is often a very critical process with regard to metering the liquid to be analysed and, thus, the reproducibility of the measurement. Such test elements require additional devices such as channels, membranes, papers or fleeces to transport and distribute the liquid samples. This design often means that relatively large sample volumes are required to enable reliable measurements. In the case that for example blood is used as a sample liquid, blood collection is all the more painful for the patient the more blood has to be collected as the sample liquid. Hence, the general goal is to provide test strips which require the smallest possible amount of sample material. Furthermore, the liquid transport should be as rapid as possible to achieve the shortest possible measurement times.
When using fleeces, papers or membranes for liquid transport, the rate of transport is decisively influenced by the properties of the respective material and hence it is not possible to guarantee uniformly high transport rates. Furthermore, major disadvantages of the aforementioned materials are that they have a not inconsiderable intrinsic volume and are themselves capillary active due to their microscopic structure.
Thus, especially fleeces and papers have a large capillary active volume due to their fibre structure which, although enabling the distribution of liquid within the material and transport from the sample application area to the detection element as a result of capillary forces, also results in retention of a not inconsiderable portion of the liquid to be examined. Consequently, a considerable portion of the originally applied sample liquid is not available for the actual detection of the analyte in such test elements so that larger sample volumes have to be used which in turn have the above-mentioned disadvantages for the patient. Furthermore, when several detection elements are arranged behind one another on a common test element the problem occurs that the detection reactions do not start uniformly and simultaneously in the detection elements. Due to the relatively slow liquid transport through capillary active fleeces, papers or membranes, the detection reaction begins considerably sooner in the detection element facing the sample application area than the detection elements that are behind it in the direction of flow. This also applies similarly to the respective detection element itself. In this case the wetting with liquid and thus the start of the detection reaction firstly occurs at the side facing the sample application area so that delays in the start of the detection reaction can also occur within a detection element. Thus non-uniform and non-reproducible reaction time courses and thus erroneous analyte determinations can occur.
In the case of several detection elements arranged one behind the other, carry over of reagents from one detection element into neighbouring detection elements which are behind it in the direction of flow can additionally occur and thus falsify the result of the measurement.
If channel structures are used to transport liquid samples from the sample application area to the detection element, certain minimum and maximum dimensions with regard to width, height and length and surface properties of the channel have to be adhered to in order to enable capillary transport of the liquid. This again puts constraints on the liquid volume to be transported and the transport rate.
Other mechanisms and devices for liquid transport often require the use of active external forces such as pumps thus necessitating additional and hence costly devices.
The channels that have been previously used in test elements often have the disadvantage that they have a not inconsiderable internal volume which, due to capillary activity, retains a portion of the liquid volume to be examined. Thus, also in such test elements a portion of the originally applied sample liquid is not available for the actual analyte detection. As a consequence larger sample volumes have to be used which again has the above-mentioned disadvantages for the patients.
Channels that have previously been used in test elements are usually composed of inert materials that are impermeable to liquids. Although a rapid capillary liquid transport can occur in these channels in the area of the detection element, further structures and devices are necessary to transport the liquid from the capillaries into the detection element.
One method is to integrate the detection element or parts thereof in direct contact with the inner space of the capillary channel such that the detection element is itself a component of the capillary channel. However, this has the disadvantage that sharp changes in the surface properties of the two materials can occur at the sites of transition from the wall of the capillary channel to the detection element which can hinder or completely disrupt the transport of liquid. Hence, it is not possible to guarantee a uniform and simultaneous flow of liquid to be examined to the detection elements. Instead, the wetting and thus the detection reaction occurs earlier in the area of the detection element that faces the capillary space or the sample application area than in sites that are further removed and, hence, it is not possible to achieve a controlled and uniform detection reaction process and a reproducible determination of the analyte.
Test elements in which capillary active materials such as fleeces or similar materials and, in particular, so-called spreading fleeces or fabrics make the junction between the capillary channel and the detection elements, are subject to similar problems and additionally have the aforementioned disadvantages of fleece-like materials for liquid transport.
EP-A-0 287 883 describes a test element that utilizes a capillary interspace between the detection layer and an inert support for volumetric metering. In order to fill the capillary space the test element is dipped in the sample to be examined requiring large sample volumes which is why this type of volumetric metering is more suitable for examining sample material such as urine which is present in excess. In this case, the capillary space is only used for volumetric metering. A spatial separation of the sample application area and site of detection and a directed liquid transport to the site of detection caused by a capillary gap is not provided in the described device. Furthermore, in the described device the detection element itself forms part of the capillary space.
DE 197 53 847 describes a test element for determining an analyte in a liquid which has a channel capable of capillary liquid transport and a detection element on an inert support. The channel capable of capillary liquid transport is characterized according to the invention in that it is at least partially formed by the support and the detection element and extends in the direction of capillary transport from the sample application opening at least to the edge of the detection element that is nearest to the vent hole. A particular disadvantage of this embodiment is that the detection element is a direct component of the channel capable of capillary liquid transport. As a result, the different surface properties of the individual components of the channel can cause the above-mentioned problems such as impairment or interruption of capillary transport or an irregular wetting of the detection element. Furthermore, the liquid to be examined in the channel itself comes into direct contact with the reagents of the detection element and hence in this embodiment there is no spatial separation between the transport space and detection area.
A device for analysing biological fluids is known from DE-A 31 51 291 which comprises a support with a self-filling measuring channel and a laminate arrangement with a filter layer and a reagent material layer. In this test carrier the sample liquid is transported into the test channel by capillary forces and from this channel it enters the laminate located above it where a detection reaction of the target analyte takes place after heating the analytical device. In this device a microporous membrane having pore sizes of less than 1 μm forms the upper cover of the capillary channel as a filter layer. According to the invention this filter membrane has the function of isolating the reagent material layer from interfering components such as cell structures. The main object of this filter membrane is to process the liquid sample before it is analysed in the reagent material layer and change its composition and it is thus already involved in the detection of the analyte. A disadvantage of this is that the very small pore openings of less than 1 μm only allow a very slow penetration of the liquid into the reagent material layer resulting in long measuring times. In particular, when using solutions such as blood which contain particles or cells in high concentrations, the filter membrane can easily become clogged due to the small pore size of the membrane and thus the transport of the liquid to be examined into the detection area can be impaired or interrupted. Consequently, it is not possible in all cases to ensure that the analyte determination can be carried out and is reproducible. Another disadvantage is that the analytical device with the sample contained therein has to be heated in order to determine the analyte. Hence, the use of the analytical device is essentially confined to laboratories.
DE 196 29 657 describes a diagnostic test carrier which contains one or more detection layers on a support layer and a network covering the detection layers which is larger than the detection layers and is attached to the support layer. In order to determine analytes the liquid to be examined is directly applied to the net and flows through it into the detection layers. In this case the sample application area and detection layers are arranged above one another in a vertical axis which results in the aforementioned problems associated with such stack-like arrangements. Before the determination of the analyte there is no specific transport of the liquid to be examined from the sample application area to a distant detection layer in a horizontal direction, for example, by means of capillary channels. Here, the purpose of the network is to transport excess liquid through the network away from the detection layer into the parts of the network extending beyond the detection layer. For this purpose the thickness of the network should be such that the cover on top and the underlying support layer are at such a distance from one another that remaining liquid over the saturated detection layer and in the filled meshes of the network is imbibed by capillary forces into the area under the cover and is led away from the sample application area. In these areas the liquid transport is in a lateral direction due to capillary forces within the network itself or capillary forces between the network and cover or support layer but not capillary forces in which the network forms a wall of a larger capillary gap. Since the network in the described device must fulfil other requirements, it also has different geometric parameters and material properties compared to the network of the present invention.